The invention is in the field of imaging systems for use in the printing industry. More particularly, the invention relates generally to the field of edge detection of imageable printing plates mounted in or on an internal or external drum platesetter used for imaging printing plates. The invention is also suitable for use in an imagesetter used for imaging a film media, which is thereafter used in the process to image a printing plate.
A critical step in a process to transfer an image to a printing plate mounted on a platesetter, for subsequent use on a printing press, is obtaining precise alignment between the image and the plate. An image, e.g. a family portrait, can be skewed if it is not precisely aligned with the outer edges of the printing plate. To prevent skewing, the outer edges of the image should be aligned with the outer edges of the printing plate. Many printing presses have registration pins for installing the plate onto the press. Often the plate has a series of holes punched into it (i.e. a collinear array of holes at each end of the plate) so the plate may be placed over the registration pins on the press. This is done so as to duplicate the same precise alignment of the plate on the printing press as when the plate was exposed to the image on the platesetter. When holes are punched into the plate, precise alignment between the holes and the outer edges of the plate is also required.
An alternate method of installing and aligning (known as registering) plates onto printing equipment e.g. platesetters, printing presses etc. is to simply place an outer edge of a plate up against the registration pins. The outer edges of the plate are then determined by various means and the image area defined with respect to the outer edges of the plate. Alignment errors are directly proportional to how accurately and how precisely edges of the plate can be determined. Various methods are employed to detect an edge of a plate. These methods include mechanical switches, optics, and various other electrical sensing techniques. Each technique has unique disadvantages. For example, mechanical switches cannot detect the edge of a plate with the same resolution that is used to create the image e.g. on the order of pixels. This limits the ability to maximize the available image area of the plate. Further, mechanical edge detection techniques can damage a portion of the plate.
Some optical techniques have been investigated, but can be limited for many reasons. For example, plates can be very thin, often on the order of 0.006 inches thick. This creates a difficult task of measuring the difference in round trip propagation time of a light pulse traveling to the plate and back versus the round trip propagation time of a light pulse traveling to the surface supporting the plate and back. The type of equipment supporting the plate also places limitations on optic techniques. Often the support surface is a metal drum, onto which the plate is mounted. The metal drum, opaque to light, makes an optic transmission method expensive to manufacture. A source must be positioned outside the drum, and a light detector placed in a recess formed in the surface of the drum. This technique is more difficult to implement on a rotating drum.
Reflective methods employed by some workers rely on differences in contrast between different surfaces to reflect varying amounts of light.
Alternatively, attempting to rely on differences in projected focal area between different surfaces to reflect different amounts of light can be difficult. Consider that the amount of light reflected from a surface will vary depending on the size of the light spot (focal area) on the surface. A large spot, with lower light density, reflects less light toward a remote point, than a small spot with higher light density does. A thin plate mounted on a surface produces a very small difference in focal area (spot size) when the spot is on the plate verses the surface. Consequently, the difference in reflected light is very small and difficult to detect.
The difference in reflected light is what makes detecting the edge of a plate possible. Large differences in the amount of reflected light between any two surfaces, simplify, or even eliminate the need for analog signal conditioning circuitry, and allow detection of very small physical discontinuities such as the edge of a thin plate. If the reflectivity between two surfaces is sufficiently different, a large difference in the amount of light reflected from each surface will result even though the physical difference in height is very small, or co-planar. An example is a piece of white paper next to a piece of black paper. The white paper reflects a large amount of light, where the black paper absorbs a large amount of light. Consequently, the black paper reflects less light compared to the white paper. The challenge is obtaining an adequate difference in reflectivity between a plate and a surface that supports the plate. When the difference in reflectivity between two surfaces is very small, e.g. between a black surface and a dark blue surface, it can be very difficult to determine a difference. The tiny difference is xe2x80x9csmearedxe2x80x9d and even often obscured by noise.
Workers have experimented by coating a surface to reduce reflected light by painting the surface with black paint. This technique may be useful to detect a plate surface, if one is willing to accept the additional complexity to integrate the reflected signals over a period of time. This technique has not worked well to detect the precise edge of a plate. One reason is the very smooth black surface still reflects some light which manifests itself as noise in the light detector. This noise reduces the signal to noise ratio of the electrical signal which is proportional to the difference in reflected light between the plate and the surface. Other factors that contribute to lower Signal/Noise ratios (the difference between a signal and noise) are variations in the 1) reflectivity of printing plates due to different manufacturing processes used by different manufacturers, 2) reflectivity of the drum (or other support surface) due to different surface treatments and debris, 3) reflectivity of the drum (or other support surface) due to surface roughness, and 4) use of thin plates produces little change in light spot size.
A technique for detecting an edge of a printing plate, and any associated skew, is disclosed in co-pending U.S. patent application Ser. No. 09/571,674 by Tice et al, and Ser. No. 09/573,638 by Wolber et al. Tice and Wolber employ a method of making a series of optical transmission measurements using a plurality of light sensors and light detectors. Edge detection sensors according to Tice et al and Wolber et al are not implemented on a drum, but in the loading and/or unloading paths to and from a drum.
An edge detection system employed on a non-rotating internal drum imaging systems is described in U.S. Pat. No. 5,889,547 to Rombult et al, and U.S. Pat. No. 6,097,475 to Jakul. Both patents describe a light detector recessed into an imaging drum for making a transmission measurement. Such a configuration is unsuitable for an external drum imaging system since an external drum rotates and would require slip rings to carry electrical signals to and from the drum.
What is needed, is a method to further reduce the amount of light reflected from an external drum or other support surface.
Further, the mechanical integrity of the edges of the plate must not be compromised and the edges must remain co-planer with the other portions of the plate e.g. not curled up or bowed down. Further, since the plate is often mounted in a stationary position, edge detection apparatus must be capable of moving with respect to the stationary plate.
It is an object of the invention herein to provide a reliable method and apparatus for precisely detecting an edge of an imageable printing plate or other imageble media mounted on a drum or other support surface of an internal or external drum imagesetter or platesetter.
It is a further object of the invention herein to increase the focal area (spot size) of a light beam by using at least one groove formed into a drum or other support surface where said focal area (spot) is inside of said groove so the amount of light reflected from a surface is reduced.
It is another object of the invention herein to decrease the amount of light reflected from a drum or other support surface using a groove formed into a drum or other support surface where the groove has anti-reflecting (light absorbing) layer deposed on an inside surface of said groove.
It is another object of the invention herein to use a specially shaped groove formed into a support surface of a drum to redirect light originating from a light source away from a light sensor responsive to light from said light source.
It is another object of the invention herein to provide a method for detecting a skewed plate or other imageable media mounted on a support surface of a drum.